I have 3.5 quarts of Castrol 5W30 and 1.5 quarts of Valvoline 5W30. Is it okay to mix them when I do an oil change?

I've done it.  I felt very weird about doing it, but it didn't hurt anything.

I would say mix them and go.  Next oil change, put your favorite oil in and forget you ever did it.

Cwkick

I'd say you'd be ok, I'm not one to say that oil is oil no matter what, like if you said Castrol and a quart of Wally World $.99 special, then I'd say maybey not.  I think the big difference in oils is the aditives and cleaners and stuff like that.  (But like Cwkick said, you do feel kind of weird)    ;DDAve

It can be a bad thing, different oil makers
put different amounts of the anti wear and
cleaning agents and so forth, if one is quite
a bit different than another, it could reduce
one of the additive packages effectivness,
but more than likley, you would never notice
any additional wear or reduced cleaning

Wild

Probably better to mix brands than weights... not that I'd know about the latter *shifty eyes*

Mixing weights is one thing, brands is another. I think the only difference between brands, is the color turban of the guys who blessed it. Pennzoil they wear yellow, Quaker State-green and Mobil- blue and so on and so forth. But heres a good one for ya: if gas prices fluctuate because of oil prices, how come the price of a quart of oil doesn't? You know, one day you're in NAPA and a quart of Castrol is $1.59, the next day its $1.79, and you're like, Man, should have gotten oil yesterday. I know, its the prices of refining the oil into gasoline that make it fluctuate, but ask someone that once, and they'll give you weird looks

Mixing brands of oil is completely safe.  As long as you don't mix different wieghts.  I mean you don't always buy gas from the same gas station now do you?

I mean you don't always buy gas from the same gas station now do you?

You mean you don't

Must be a habit I picked up from back home when there was only one gas station per town

 Information for this article was compiled from reports and studies by the University of Nevada Desert Research Center, DuPont Chemical Company, Avco Lycoming (aircraft engine manufacturers), North Dakota State University, Briggs and Stratton (engine manufacturers), the University of Utah Engineering Experiment Station, California State Polytechnic College and the National Aeronautics and Space Administration's Lewis Research Center.

Oil additives should not be used. The oil companies have gone to great lengths to develop an additive package that meets the vehicle's requirements. Some of these additives are synergistic, that is the effect of two additives together is greater than the effect of each acting separately. If you add anything to the oil you may upset this balance and prevent the oil from performing to specification.

Though some additives may not contain anything harmful to your engine, and even some things that could be beneficial, most experts still recommend that you avoid their use. The reason for this is that your oil, as purchased from one of the major oil companies, already contains a very extensive additive package.

This package is made up of numerous, specific additive components, blended to achieve a specific formula that will meet the requirements of your engine. Usually, at least several of these additives will be synergistic. That is, they react mutually, in groups of two or more, to create an effect that none of them could attain individually. Changing or adding to this formula can upset the balance and negate the protective effect the formula was meant to achieve, even if you are only adding more of something that was already included in the initial package.

This information should also be taken into account when adding to the oil already in your crank case, or when mixing oils for any reason, such as synthetic with petroleum. In these cases, always make sure the oils you are putting together have the same rating (SA, SE, SC, etc.). This tells you their additive packages are basically the same, or at least compatible, and are less likely to upset the balance or counteract each other.

http://www.vtr.org/maintain/oil-additives.html
http://www.vtr.org/maintain/oil-overview.html